Modern elevator systems have doors to permit transfer of passengers between the elevator cars and the respective floor landings. Because smaller doors have to travel a lesser distance and have less inertia, many elevators have two doors. They may meet in the middle, and thereby have a lesser distance to travel or they may both travel to the same side for opening. Other elevators may have only a single door. As used herein, the term "door" or "doors" may be used interchangeably, it being understood that there is no distinction between a single door and double doors concerning the subject matter hereof.
Present day elevator systems have doors mounted on the elevator car, and doors mounted at each hall landing of the elevator hoistway. The hoistway doors at the hall landings are mounted directly to the building structure, and are kept closed whenever the car is not present at the related landing in order to prevent passengers and objects from entering the hoistway. Instead of having door operators for each of the hoistway doors, the hoistway doors are typically opened by coupling them with the car doors, so that opening of the car doors will open the landing doors in unison therewith, thereby protecting passengers in the car from the building structure and protecting passengers at the landing from the hoistway.
The manner of coupling the doors together must take into account several factors. The doors usually begin to open just before the car reaches the landing (such as 10 or 15 centimeters therefrom), resulting in relative vertical motion between the elevator door and the hoistway door as the elevator approaches the landing. A similar constraint is that the car may be releveled after the doors are open, which also requires permissible relative vertical motion between the car doors and the hoistway doors. The hoistway doors may easily be pushed open by the elevator doors, but they must also become closed, either by being pulled (or pushed) toward the closed position by the elevator doors, or by some biasing in the closed direction. Biasing in the closed direction may take the form of a spring, a weight or aspirator. However, any bias in the closed direction must be overcome by the force exerted by the elevator doors during the opening process. Therefore, such bias is usually limited to a minimal amount necessary to meet self-closure requirements in government codes. Similarly, any perturbations in the bias during the opening process will in turn provide perturbation in the control algorithm for the elevator door opening system. Therefore, it is deemed preferable to have the hoistway doors opened and closed by the elevator doors, without separate bias.
A typical coupling device employs a rigid vane mounted on the car door which engages a rotatable pawl on the hoistway door, the pawl having rollers thereon so that the vane can travel upwardly or downwardly while engaging the pawl. Typically, there may be some lost motion between the two doors; that is, the car door must begin to open before it engages the pawl, unlocking the hoistway door, and commencing to push the hoistway door, through the pawl, in the open direction. When closing, the hoistway doors must be fully closed before the car door motion stops (before the car doors are fully closed). In some assemblies, the rollers move into contact with the vane before motion, and in others, the vane is expanded to contact the rollers before any motion. However, devices of this type are wear and adjustment sensitive and require frequent adjustments and replacements over the life span of an elevator system.
Whenever there is a change in the amount of force required to move an elevator car door, either because of lost motion between it and a hoistway door, or because of a change in the mechanism leverage and the like, perturbations can result within the electrical control system which provides the motive force for the car door opening mechanism. This in turn can cause vibrations and other mechanical perturbations, thus resulting in additional wear and noise. And, in very tall buildings, door closing (particularly at the lobby) can be erratic due to hoistway air pressure (called "windage" or "chimney effect"), unless the hoistway doors are closed positively.
Of course, any coupling mechanism located on a particular hoistway door must have complete clearance, for all of the apparatus, including the corresponding parts of a coupling device which are mounted on the car door, so that elevators that are simply passing by landings do not run the risk of contact with the hoistway door coupling devices.